The number of conventional, volatile refrigerant fluids available for use is steadily being reduced by legislation and general concerns arising from the apparent contribution to ozone layer depletion by certain widely used refrigerants; and, further, the contribution of such refrigerants to global warming due to the "greenhouse" effect.
Accordingly, there is a need to replace or reduce the use of such refrigerants with fluids which have no, or at least minimal ozone depleting potential and do not contribute unduly to the greenhouse effect.
One possible means to reduce the use of such potentially harmful refrigerants is to use a relatively small volume of volatile primary refrigerant to cool a much larger volume of secondary heat transfer liquid which may be pumped to the locations where cooling is required. In this way the total volume of refrigerant is reduced, since the distribution of cooling to the desired locations is accomplished using the secondary heat transfer liquid. This approach is commonly used in air conditioning systems which utilise water as the heat transfer liquid. Where cooling below freezing point is required, various aqueous solutions such as propylene glycol, ethylene glycol and calcium chloride brine may be employed as the secondary heat transferring liquid. At still lower temperatures, the viscosity of these aqueous solutions rises, requiring the use of liquids such as trichloroethylene. However, substances such as trichloroethylene tend to be toxic and are frequently highly corrosive.
U.S. Pat. No. 4,344,296 (Staples) discloses a two-stage refrigeration system having a primary vapour-compression refrigerant circuit, and a secondary heat transfer circuit. The secondary heat transfer circuit comprises a condenser (which is cooled by the evaporator of the primary refrigerator) and a secondary evaporator which provides cooling at a desired location. Ammonia is proposed as the refrigerant for the primary refrigeration circuit. However, ammonia is potentially hazardous. The provision of a secondary circuit allows a less toxic refrigerant to be used. Refrigerant 22 is proposed as the secondary heat transfer fluid. However, refrigerant 22 is a fluorocarbon refrigerant having some ozone-depletion and global warming potential.
U.S. Pat. No. 3,683,640 similarly provides a secondary refrigeration circuit which utilises a mixture of volatile heat transfer fluids which is cooled by an Electrolux-type primary refrigerator. A mixture of refrigerant 12 and refrigerant 13 is proposed as the volatile secondary heat transfer fluid. These refrigerants are also environmentally unacceptable fluorocarbons.